I have tint on my Carrera and it does make a difference. On my BMW I had it tinted with the lightest the shop had in front and their mid level in back. The change was very noticeable since the interior is black leather. A good shop will charge about $200 for a sedan. It's worth having it done by a pro.
Ed - I found out about the new law a week after I got pulled over in a small town down state late one night. Cop pulled me over for no other reason than the tinted front windows. This was 2 yrs after I had it done. Around town (home) the cops here have more important thinks to do than harass a middle age punk... There are plenty of younger punks. Anyhow, never was I pulled over for tint until I was in the road. I go a warning for it, but the timing was great. I was Nov 7th last year, 7 days after the new law. Small town cop was still handing out a warning ticket form with "window tint" as a selection. A week later I found out the warning wasn't even allowed
